Conventionally, for scanning line interpolation which creates a progressive video image from an interlaced video image in order to upcovert a television system into another with a different number of scanning lines or in order to render an image at a higher definition, intraframe interpolation is performed for still images. For motion pictures, intra-field interpolation is performed because the correlation within a frame may corrupt in a motion picture and defective conversion such as jaggedness may occur when moving vertical lines are interpolated with the frame.
As simple methods of intra-field interpolation, lines to be interpolated may be processed by line interpolation with the lines directly above or may be processed by line interpolation using the average of the lines above and below. The former interpolation has been accompanied by jaggedness in image outlines for patterns such as inclined lines etc., having less correlation with respect to the vertical direction. The latter interpolation method has been accompanied by image degradation such as unclearness etc., in the image.
As a technique for solving these drawbacks, the interpolation method of Japanese Patent Application Laid-Open Sho 63 No.187785 discloses interpolation whereby every pixel on lines to be interpolated is interpolated according to the information of the pixels which reside around the pixel and in a direction in which the pixel to be interpolated has the strongest correlation. First, in order to obtain information as to the radial direction in which the pixel to be interpolated has the strongest correlation of pixel data, absolute values of the difference between neighboring pixels with respect to the vertical direction and right and left diagonal directions are checked so as to determine that the direction along which the absolute differential value becomes minimum is the direction presenting the strongest correlation, whereby the average value of the pixels in that direction is determined and allotted to the value of the pixel to be interpolated.
However, this method needs to calculate the absolute differential values at least in the three directions as above and determine the minimum value of these absolute differential values to determine the value of the pixel to be interpolated for each of all the pixels constituting the lines to be interpolated. Therefore, it takes a long time for interpolation processing. Further, since the above sequence of processing is also performed for all the area including pixels other than at the edges in the image(where the pixel values vary little) or for the cases where there is little difference in the degree of correlation depending on the directions, the time for processing is wastefully consumed, hence the speed of interpolation is low so that it is also difficult to enlarge the range of pixels of which the correlation is checked.
Therefore, when interpolation is implemented for an image including an inclined line having a small inclination where pixels located considerably apart from each other have correlation, it is impossible to perform interpolation based on the strong correlation between these pixels, sill leading to production of jaggedness along the edges of the inclined portion with poor interpolation precision.
In order to overcome the drawback of the above publication, Japanese Patent Application Laid-Open Hei 5 No. 30487 discloses a method which improves the processing speed of interpolation and improves the interpolation accuracy by enlarging the range of searching for correlation with a corresponding enhancement of the processing speed.
Next, the image interpolation method disclosed in this publication will be described with reference to FIGS. 14 to 17.
To begin with, in this interpolation method, as shown in FIG. 14, the pixels on neighboring lines, namely n-th line and (n+1)-th line, on a two-dimensional image are compared so as to detect edge points a and b on respective lines in the two-dimensional image. It is assumed that the pixels which are on the line to be interpolated, other than the points between the edge points a and b, may be interpolated by the pixels on either of the adjacent lines so as to determine the edge on the line to be interpolated.
Next, as shown in FIG. 15, around the edges a and b, a neighboring pixel row (3,0), which is centered at an observed pixel(pixel A in this case) on either of the neighboring lines(n-th line) and is defined by ‘the number of pixels’ and ‘the amount of shift from the position to be interpolated’ is created while an associated pixel row (3,1) correlated with the neighboring pixel row is selected from the other adjacent line(n+1)-th line.
Here, as a general form for establishing correlation between the neighboring pixel row and the associated pixel row, the neighboring pixel row is defined to be (2 m+1,0) and the associated pixel row is defined to be (2 m+1, ±Y) where m and Y are sequentially changed as m=Y=1, 2, 3, 4, 5.
Next, as shown in FIG. 16, based on the operations between each pair of pixels indicated by the arrow in the drawing, in neighboring pixel row(3,0) and associated pixel row(3,1), whether the level difference between the pixels falls within the predetermined range is checked so as to determine the correlation. In the case shown in the drawing, no correlation exists between the centered pair of pixels. Hence, m and Y are set equal to 2, the operations between each pair of pixels, indicated by the arrows in the drawing are performed for the neighboring pixel row (5,0) and associated pixel row (5,2). When the presence of correlation is recognized from the pairs of pixels indicated by the arrows in the drawing, the amount of shift from the position to be interpolated is known to be two pixels. That is, as shown in FIG. 17, point a and point b are two pixels shifted from one another and hence interpolation is implemented using a pixel row, which is created by shifting the selected pixel row or the neighboring pixel row by half the determined number of pixels in the direction opposite to the shifted direction, specifically, the pixel row centered by point c, which is one pixel shifted rightward from pixel A.
The interpolation method disclosed in Japanese Patent Application laid-Open Hei 5 No. 30487 is an invention which adopts interpolation on a pixel row basis and provides improved processing speed compared to a typical interpolation on a pixel basis. However, in order to determine the edge of the pixel row, arithmetic operations between lines should be performed for the rows of at least three pixels to, more or, less, eleven pixels while shifting pixels around the edge vicinity, hence the aspect of the long time requirement for arithmetic operations is still unsolved in this method. Further, this interpolation method is aimed at smoothing the outlines of images hence has the problem of inability to restore a pattern if the pattern has gaps at inter-lines as in an image depicted with nearly horizontal fine lines.
In view of the above problems of the conventional art, the object of the present invention is to provide an image interpolation system and image interpolation method which can realize real-time interpolation of various video pictures including patterns with fine lines, edges etc., by sharply reducing the processing speed.